This invention relates to the calcining of materials of a tacky consistency having a propensity for adhering. The invention is particularly directed to rotary cylinder calcining apparatus for calcining "sticky" materials such as ammonium diurnate in the manufacture of uranium dioxide for fissionable fuel used in power generating nuclear reactors.
The present invention comprises an improved calcining apparatus which is useful in manufacturing processes and for producing products of the type described in U. S. Pat. Nos. 2,906,598; 3,037,839; 3,579,311; 3,871,829; and 3,969,477, among other background patents. The disclosure and contents of said patents are therefore incorporated herein by reference. The background of this invention and the state-of-the-art relating thereto are described in some detail in the aforesaid patent disclosure.
This invention applies generally to rotary calcining systems for treating assorted materials of any composition which are of a tacky consistency whereby they exhibit a propensity for adhering to exposed portions or components treating or handling equipment. The invention is specifically applicable to rotary calcining apparatus used in the manufacture of uranium dioxide for use as fissionable fuel in power generation as described in the aforesaid patents.
As is evident from the above cited U.S patents, a common method of producing fuel grade uranium dioxide comprises the hydrolysis of uranium fluoride gas in water to form uranyl fluoride, which material is treated with ammonium hydroxide in solution to precipitate out ammonium diurnate. The ammonium diurnate is then decomposed by heat to uranium dioxide in a calcining operation wherein residual water and any resultant gaseous by-products of the heat treatment are expelled. The consistency of the ammonium diurnate precipitate from this process varies somewhat with respect to the conditions of its formation reactions, for example concentrations and pH of the reacting solutions, temperature, and solution contents such as amounts of fluoride and other possible ingredients of the solutions involved in the process. In any case, the precipitated ammonium diurnate product is typical of a generally viscous and sticky consistency which is prone to adhering or clinging to any surface with which it may contact. In some instances this precipitate has been aptly described as having the consistency of "peanut butter", or a "putty-like sludge".
In accordance with established processes and the procedure in this art, the precipitated ammonium diurnate is partially dewatered by conventional means, and then calcined as described in the above cited U.S. patents. However, the typical tacky consistency of the ammonium diurnate precipitate product and the resultant propensity for its adhering to equipment has created production difficulties and problems which have been unusually costly and cumbersome to cope with in some manufacturing operations. The adherence of such materials very commonly occurs within the confines of the rotary calciner in such critical locations as the restricted area of the outlet therefrom, a portion of the system which is highly prone to blockage and wherein the necessary constant outward flow of solids and gases for continuing production is frequently obstructed.
This problem of adherence and blockage attributable to the tacky consistency of material within fuel producing equipment is especially troublesome and expensive because of the typically inaccessibility of the location of adhering material within a system and the frequency of blockage resulting therefrom, as well as the high temperatures involved within the calcining apparatus. Also, when treating enriched nuclear fuel materials, radiation precautions must be utilized for personnel dealing with the problem.